1. Field of the Invention
The present invention relates to an aqueous ceramic slurry composition, a ceramic green sheet using the composition and a multilayer ceramic electronic part using the ceramic green sheet.
2. Description of the Related Art
Multilayer ceramic electronic parts have been manufactured by laminating a plurality of ceramic green sheets having a thickness on the order of 10 to several tens of xcexcm to form raw ceramic laminates, followed by firing. In complying with the requirements for miniaturization, weight reduction and implementation of higher densities of multilayer ceramic electronic parts in recent years, however, there has been a demand for ceramic green sheets having a thickness on the order of 3 to 5 xcexcm.
Ceramic green sheets such as those described above are prepared by comprising the steps of, for example, dissolving an organic binder such as a polyvinyl butyral resin into an organic solvent such as a ketone, a hydrocarbon, an alcohol, an ester or an ether alcohol to form an organic vehicle, mixing a ceramic powder with the organic vehicle to form a ceramic slurry in which the ceramic powder is dispersed, subjecting the slurry to a sheet formation process to spread the slurry over a carrier tape by means of a doctor blade method, a roll coating method, a gravure coating method or the like, in order to form sheets having a specific thickness, and drying the sheets to volatize and eliminate the organic solvent from the sheets.
Use of organic solvents imposes a large burden on the working environment. Furthermore, since the organic solvents contained in a ceramic slurry evaporate quickly, there are occasions when boiling and convection of the organic solvents occur in the course of drying the ceramic slurry, generating sheet defects such as pin holes in the dried ceramic green sheets. These defects become the causes of defects in the electric properties of the multilayer ceramic electronic parts eventually prepared by firing raw ceramic laminates formed by laminating a plurality of the ceramic green sheets.
Accordingly, investigations have been made regarding an aqueous ceramic slurry composition wherein an aqueous solvent is used instead of organic solvents and ceramic green sheets are formed, or using an aqueous ceramic slurry comprising an organic vehicle in the form of emulsion in which a water-soluble resin such as an acrylic resin, polyvinyl alcohol, a vinyl acetate-ethylene copolymer or the like, is chosen as an organic binder.
However, water has a surface free energy of 72 mN/m. That is extremely high in comparison with organic solvents, and therefore, aqueous ceramic slurry compositions are inferior in wettability toward materials having a low surface free energy such as a film represented by a carrier tape, tending to cause a problem of sheet defects such as pin holes.
Furthermore, when a water-soluble resin such as a water-soluble acrylic resin or polyvinyl alcohol is used as an organic binder, it is difficult to have its molecular weight increase to a sufficient level, causing a problem in that the tensile strength and elongation of the formed ceramic green sheets are lowered and that thin-layer ceramic green sheets and multilayer ceramic electronic parts using them are difficult to be manufactured.
It is an object of the present invention to solve these problems, by providing very thin aqueous ceramic green sheets that are substantially free from sheet defects such as pin holes, that have a low level of surface roughness and that have high tensile strength and elongation, and by providing multilayer ceramic electronic parts using them.
Accordingly, one aspect of the present invention is a ceramic slurry composition comprising a ceramic powder, a vehicle comprising an organic binder and an aqueous solvent, and a surfactant comprising an alkyne diol and/or an ethylene oxide addition product of such an alkyne diol.
It is to be noted that the aqueous solvent may be water, or it may be a mixture of water with a small amount of another solvent such as an alcohol, an ether, a ketone or an ester.
It is preferable that the above-described organic binder in the ceramic slurry composition according to the present invention be polyurethane resin particles having an average particle size of about 120 nm or less. Furthermore, the vehicle is preferably in the state of emulsion wherein the polyurethane resin particles are finely dispersed in the aqueous solvent.
It is also preferable that the amount of the above-described surfactant in the ceramic slurry composition according to the present invention is about 1 part by weight or more based on 100 parts by weight of the ceramic powder.
Another aspect of the present invention is a ceramic green sheet formed into the shape of a sheet using a ceramic slurry composition comprising a ceramic powder, a vehicle comprising an organic binder and an aqueous solvent, and a surfactant comprising an alkyne glycol and/or an ethylene oxide addition product of such an alkyne glycol.
It is preferable that regarding the ceramic green sheet according to the present invention, the above-described organic binder consists of polyurethane resin particles having an average particle size of about 120 nm or less. It is also preferable that the vehicle is in the state of emulsion wherein the polyurethane resin particles are finely dispersed in the aqueous solvent.
It is also preferable that the amount of the above-described surfactant in the ceramic slurry composition is about 1 part by weight or more based on 100 parts by weight of the ceramic powder.
Accordingly, it is possible to manufacture a very thin ceramic green sheet having a thickness in the range of about 0.5 to 5 xcexcm.
Still another aspect of the present invention is a multilayer ceramic electronic part comprising a ceramic laminate formed by laminating a plurality of ceramic layers and a terminal electrode formed in such a way that it contacts the ceramic laminate, wherein the ceramic layers are formed by firing the ceramic green sheets according to the present invention.
Further objects, features and advantages of the present invention will become apparent from the following description of the preferred embodiments with reference to the attached figures and tables.